Role of Chain Flexibility in Asymmetric Polyelectrolyte Complexation in Salt Solutions

Complexation of oppositely charged macromolecules is essential in several biological phenomena. Recent reports have demonstrated the importance of DNA local flexibility in governing liquid-liquid phase separation (LLPS), a ubiquitous phenomenon thought to drive membraneless cellular organization. Inspired by this, we perform coarse-grained molecular dynamics simulations to study the role of chain flexibility in the complexation of short negatively charged polyelectrolytes with long positively charged polyelectrolytes. At low ionic strengths, spontaneous segregation of chains into a condensed (polymer-rich) phase and a supernatant (polymer-depleted) phase is observed. When both the polyanion and polycation are flexible (flexible-flexible complex), denser complexes with a higher degree of structural correlation form, with fewer free chains released into the supernatant, compared to the case when the polyanion is rigid (rigid-flexible complex), in agreement with the LLPS experiments. Free chains are in rapid exchange between the supernatant and the condensed phase. The partitioning of salt ions in the two phases depends on chain flexibility, with salt ions partitioned more into the condensed phase for flexible-flexible complexes. Interestingly, at intermediate to high salt concentrations, flexible-flexible complexes form multiple equilibrium finite-size clusters, suggesting nanophase to microphase segregated structures, while rigid-flexible complexes tend to condense into a single complex. The results provide molecular-level insights into LLPS of asymmetric polyelectrolyte complexation and give guidelines for assembling oppositely charged macromolecules with different degrees of molecular flexibility.